(1) Field of the Invention
The invention relates to the general field of field emission arrays with particular reference to how the enclosure is sealed and metal leads brought out from the interior vacuum.
(2) Description of the Prior Art
Field Emission Arrays (FEAs) are most commonly packaged between flat glass plates. The cathode lines, microtips, and (orthogonal) gate lines are formed on one plate (the rear plate) while the fluorescent screen, which also acts as the anode, is formed on the other plate (the front plate). To control the gate to anode separation, glass spacers, located along the outer edges of the plates, are placed between them and then sealed to the plates by means of glass frit. The assembly is then given a suitable heat treatment so that the frit fuses and bonds to the plates and the spacers, after which it is allowed to return to room temperature.
During subsequent processing the inside of the FEA assembly is evacuated to a very high degree of vacuum (generally better than about 1 microtorr) and permanently sealed. Because the effectiveness of the field emitting microtips is readily degraded by the presence of gaseous contaminants, it is essential that the initial vacuum be maintained within the FEA enclosure throughout its operating life. To this end, standard gettering techniques are used but, if a very slow leak is present, the getter will eventually be saturated and performance of the FEA will start to degrade. Since this type of problem may take substantial time before it manifests itself, testing at the factory may not identify its presence prior to sale of the product.
Referring now to FIG. 1, we show a schematic view of an FEA enclosure. Front plate 1 is seen to be mounted on rear plate 2 with spacers 3 located between them. Fused glass frit 4 is seen as forming the bond between spacers and plates. Also shown is conductive lead 5 which passes from inside the enclosure (i.e. the vacuum) to the outside (i.e. the air). On the inside, 5 would normally be the termination of a cathode or gate line while on the outside it would normally be attached to a flexible lead of some sort. In the prior art, 5 has been a single continuous line of a single material. The glass frit to which 5 bonds (designated as 14 in the figure) has the same composition as the frit 4 used at other locations.
For reasons relating to the performance requirements of FEAs the preferred materials for making leads such as 5 have been molybdenum and niobium. These refractory metals have low coefficients of thermal expansion and are therefore not a good match for the relatively high expansion coefficient glass frits. This mismatch in expansion coefficients can lead to microcracking at the metal-glass interface and/or open circuiting of lines such as 5. It is not possible to use frits having lower expansion coefficients because this would raise their softening temperatures to unacceptably high values.
A number of vacuum seals suitable for use with FEAs have been described in the prior art but none is entirely free of the above described problems. Thus Kane et al. (U.S. Pat. No. 5,157,304 October 1992) teach use of a special interface layer that is first formed on the rear plate to facilitate bonding between the two plates with continuous wire leads passing directly over, and resting on, this interface layer. In one embodiment of their invention, the FEA is formed on a silicon substrate (as opposed to the rear glass plate itself) and low resistance (doped) regions are formed in the silicon for the purpose of underlying their interface layer, presumably with a view to minimizing any loss in planarity.
Chirino et al. (U.S. Pat. No. 4,293,325 October 1981) describe the formation of high temperature hermetic seals suitable for joining ceramics. These seals are based on glass frit compositions but have a high metallic content. Thus they are cermets rather than glasses and are poor electrical insulators.
Mariani (U.S. Pat. No. 5,059,848 October 1991) describes a vacuum tight package for a SAW (surface acoustic wave) device. Unbroken bus bars of uniform composition run out of the vacuum, through the seal, out into the air.
Hertz (U.S. Pat. No. 5,195,019 March 1993) teaches the encapsulation of a capacitor stack by first coating it with glass frit and then fusing the frit. To make contact with the capacitor's two electrodes, wires are attached to these electrodes prior to application of the frit. These wires protrude through the frit and become bonded to it when it fuses.